Time of flight ranging systems, are commonly used in level measurement applications, and referred to as level measurement systems. Level measurement systems determine the distance to a reflector (i.e. reflective surface) by measuring how long after transmission of a burst of energy pulses, an echo is received. Such systems typically utilize ultrasonic pulses, pulse radar signals, or microwave energy signals.
Time of flight ranging systems are commonly utilized in remote locations where process variable data is transmitted to a central location for further processing or collection, for example storage tanks and vessels in a petro-chemical plant. A common means for transmitting such data is by a current loop. The value of the process variable is represented by the magnitude of a current passing through the loop, with the magnitude lying between predetermined minimum and maximum values, typically 4 mA and 20 mA, for a 20 mA loop. Such a current loop has a high degree of noise immunity and has gained widespread industrial acceptance.
In time of flight systems, the transmitter usually has electrical power requirements of its own, and it is often convenient to meet these power requirements from the current passing in the loop. A limitation of such loop-powered transmitters has been that they must be able to operate at the minimum level of loop current, typically 4 mA. In recent years, a number of “smart” or “intelligent” transmitters have been developed, which utilize microprocessors or microcontrollers to control sensing or measurement of the process variable, and conversions of the data generated into an appropriate current level in the loop.
When the instrument is first turned on, i.e. powered up, there is an inrush of current into the level measurement instrument. In addition to inrush conditions, the circuitry may also be subjected to surges comprising a fast transient voltage impulse or transients. To protect the circuitry in the instrument, for example in an intrinsically safe application or an explosion-proof application, a 50 mA fuse is commonly provided on input port which is coupled to the 20 mA loop. Advantageously, the fuse exhibits a very low dropout voltage, if the fuse is operated in protection mode, and the inrush current exceeds the fuse rating, then the fuse will blow effectively limiting the current and requiring replacement of the fuse. It will be appreciated that for certain remote situated level measurement installations this is not practical. Moreover, in explosive environments, the circuitry is usually potted, e.g. encased in epoxy, thereby making it next to impossible to replace a blown fuse without replacing the entire instrument. It is also possible to use a resistor to limit the inrush or surge current. Advantageously, a properly rated resistor can limit the current without the need for replacement like a fuse, however, the resistor limiter can result in an appreciable voltage drop, which is undesirable, especially, in a loop-powered implementation.
In other applications, such as switching power supplies, it is also advantageous to limit the inrush current while also minimizing the voltage drop across the limiter to save power.
The inrush current is specified for various applications according to standards organizations. For example in Germany, under the NAMUR standard, the inrush current must be less than 15 times the normal peak current in steady state for level measurement instruments used in petro-chemical applications.
One known approach is to configure an operational amplifier, i.e op-amp, to function as a current source and thereby limit the inrush current. Advantageously, an op-amp circuit can be configured to be stable over temperature. Unfortunately, standard op-amp circuits are not suitable for loop-powered applications because of the voltage reference level, typically 1.25V and greater, and the high power consumption requirements.
Accordingly, there remains a need for a current limiter which provides a low dropout voltage like a fuse, but does need to be replaced if the inrush current temporarily exceeds the rated value, like a resistor, and suitable for use in a current loop application.